Separating circuit



June 24, 1941. M R, C, BALLARD 2,246,659

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R. C. BALLARD SEPARATING CIRCUIT Filed April 4, 1939 ist. Jeparanr 0 GRID vanas 0 Gnu: voL'ns .Bl/71s -v- Summer Handauaazzard Gttorncg Patented June 24, 1941 sarsas'rme cmcurr Randall C. Ballard, Audubon, N. J., assigner to Radio Corporation of America, a corporation of Delaware y Application April 4, 1939, Serial No. 265,923

(Cl. Pls-7.3)

4 Claims.

My invention relates to television receivers and particularly to circuits' for separating synchronizing pulses from picture signals.

I n present day television systems. it is the practice to transmit a composite signal consist-- ing of picture signals and synchronizing pulses as described in British Patent No. 448,065. At the receiver it is desired that the synchronizing pulses be separated from the picture signals and that these pulses only be applied to the horizontal and vertical deiiecting circuits. Various circuits for obtaining this separation have been proposed but, in practice, good separation has been diiiicult to obtain because oi variations in amplitude of the incoming signal. This dimculty could be overcome by utilizing an automatic volume control circuit having a suiilciently fiat characteristic but the use of such a circuit would be unduly expensive.

Accordingly, an object of my invention is to provide an improved circuit lor separating syn chroniuing pulses irom picture signals. I `it is o. iurther object of my invention to provide an improved means for obtaining complete separation of synchronizing pulses from picture signals regardless of wide variations in the amplitude of the signal impressed upon the separating means.

in a preferred embodiment of my invention,

, i employ two synchronizing impulse-separating circuits connected in cascade through an ampliiler, `the circuit design being such that the ilrst separating circuit functions only as an amplifier :for signals of low amplitude while complete separation is eiIected by the second separating circuit. For signals ot comparatively high amplitude,

however, the first separating circuit eiIects parembodiment of my invention, l

Figure 2 is a circuit diagram oi' another' embodiment of my invention.

ltigures 3 and 4 are diagrams showing the operation ot my circuit for incoming signals or low amplitude. A

Figures 5 and 6 are diagrams showingthe operation of my circuit for incoming signals of high amplitude, and r Figure 'l is a diagram which is referred to in explaining the operation of my improved separating circuit.

Referring to Fig. 1, there is shown a portion of `a television receiver embodying one preferred form of my separating circuit. The video signal comprising picture signals and synchronizing pulses appears in the output circuit of a second detectorindicated at Ill and is supplied over a conductor Il to the video ampliiler and cathode ray tube and over a conductor I2 to the separating circuit.

The separating circuit comprises a first separating tube I3, an amplier tube I4 and a second separating tube i6. ln this particular embodiment, the two separating circuits are oi the samey typen each one having synchronizing impulses of positive polarity impressed thereon and being biased by means oi a now of grid current. .u separating circuit of this type is described and claimed in Patent No. 2,207,839, issued July 16;, inrit, in the name oi iii?. A. IToison and assigned to the Radio Corporation of America.

@onsldering the separating tube i3, it may be a triode as illustrated or it may be of a type including more grids such as a pentode.' Signal is supplied to the grid ill oi thetube i3 through a grid condenser it. A grid leak resistor itv is provided having suiilcient resistance to prevent the grid condenser it trom discharging a very substantial amount between the occurrence of successive frame frequency synchronizing pulses.

Preferably a lower than normal plate voltage is applied to the plate 2| of the tube i3 to cause plate current cut-oir to occur at a less negative grid voltage than would otherwise.

The output of the tube I3 is applied to the amplier tube il through a coupling condenser the pme of tube i4.

will take a wide grid voltage swing without distortion but may be the same typetube as the tube i3, for example.

The second separating circuit in the circuit Aswill appear hereinafter, theampilner tube Il need not be of a type which assumed.

v Fis. 6.

the time constant of the corresponding' circuit for the preceding tube. Thus, lfrom the input to the output of the complete separating 'circuit the time constant of the grid condenser-grid leak resistor circuit is decreasing in value.

The voltage applied to the plate 3| of tube I6 preferably is below rated voltage but may be higher than the voltage applied to the plate of the rst separator tube.

The operation of the circuit will be apparent from the following discussion of the circuit action under two conditions, first, for-an incoming signal of\smal1 amplitude, and, second, for an f incoming signal of large amplitude.

Referring to Fig. 3, the grid voltage-path current characteristic of the tube I3 is shownv at 3i. An incoming signal of comparatively small amplitude is shown at 32-33, the picture signa1 portion being indicated .at 32 and'the line synchronizing pulses being indicated at 33. It will be apparent that under these conditions the tube I3 is acting asl an amplifier, its output being indicated at y32a-33a. Its output is further ampli- "fled by the tube-l4 wherebyA a signal of large enough amplitude forgood separation appears on the grid ofthe second separator tube i6 as shown in Fig. 4.

In Fig. 4, the grid voltage-plate current characteristic of the tube I6 is shown at 34%. The input signal 32-33 has sufficient amplitude to cause only the synchronizing pulses to appear in the plate circuit of the tube I 6 as indicated at 39a.

From an inspection of Figs. 3 and 4, it will be seen that the grid bias of a separating tube increases with an increase in the amplitude of the applied signal as described in the abovementioned Toison application whereby there is good separation at the second separating tube place of the iirst separating circuit. This is not the case, as will be evident from an inspection of Fig. 7, where the grid voltage-plate current characteristic of the amplifier tube I4 is shown at 36. It is assumed that an incoming signal 32-33 o f large amplitude has been applied to the grid oftube I4 without any separating action in a preceding tube: Obviously, some or all of lthe synchronizing pulses will be clipped or wiped off and the resulting signal 32a appearing in the plate circuit of the tube I4 will be worthless for synchronizing purposes.

On the other hand, by practicing my Invention, the signal applied to the tube I4 under the conditions just assumed is the signal 32a-33a (Fig. 3). It is apparent that this signal 32a- 33a may readily be amplified by an amplifier tube such as the tube I4 without distortion or clipping of the synchronizing pulses.

The diflculty illustrated by Fig. 7 could be avoided by employing a tube designed to give an undistorted output with a wide voltage swing on its grid. This, however, would be attained with lower amplification and thus insuiiicient signal would be available on week input signals.

In Fig. 1 certain circuit and voltage values have been indicated merely by way of example. In this particular circuit, the tubes I3 and I4 were the two sections of a tube of the 6N7 type. The tube I6 may be of a type having a characteristic similar to that ofone section of the 6N? tube.

As previously mentioned, the grid condensergrid resistor circuits of the several tubes preferably have time constants of decreasing values. It has been found that this feature gives the overall circuit a better low frequency response.

although there is either little or no separation.

at the rst separating tube under the condition Now considering the second condition of operation where the amplitude of the incoming signal is large, reference will be made to Figs. 5 and 6.

Referring to Fig. 5, it will be seen that the signal 32-33 of increased amplitude has caused the bias on the grid of separator tube I3 to in.

crease. Partial separation ofv synchronizing.

pulses and picture signals occurs, the signal appearing in the plate circuit of the tube I3 being as shown at 32a-33a.

'Ihe output of tube I3 is amplified by tube I4 and applied to the separator tube as shown in\ Complete separation is provided by this second separating circuit as shown at Ila-33a.

From the foregoing, the importance of applying signals of substantial amplitude to a separating circuit for obtaining goodA separation g will be appreciated. Itmight appear at first glance that satisfactory separation would be obtained by employing an ordinary amplifier in As illustrated in Fig. 2, my invention is not limited to the luse of the specific separating circuit shown in Fig. l but may comprise a first separating tube 4I having a cathode resistor i2 shunted by a condenser 43 for providing a variable grid bias due to plate current ow. The time constant of the biasing circuit 42-43 is such that most of the charge on condenser G3 holds over between successive frame synchronizing pulses. The synchronizing pulses, of course, are applied^to the grid of the tube 4I .with positive polarity.

V'I'he amplifier 44 may be the same as the ampli- Iier I4 of Fig. 1. It will be understood, however, that in some instances it may be preferable to apply a fixed bias to the grid of either one of these amplifier tubes. It may be noted that the tube 44 (also the tube I4) acts as an ampliiler rather than as a separator even though there is no xed bias since substantially full plate voltage is applied.

The second separating circuit may be of the diode type comprising, 'for example, a diode 46 having a comparatively high resistance resistor 4'I across which the signal is applied and a comparatively low impedance resistor 48 across which the synchronizing pulses only appear. The input signal is applied through a grid condenser 49 which is charged by the periodically recurring flow of diode current caused by the synchronizing pulses.

'Ihe discharge circuit cludes the resistor'4l for condenser 49 inwhich has a sulciently highvalue to prevent the condenser 49 from discharging any great amount between the occurrence of successive line synchronizing pulses. This particular diode separator is described in Poch Patent 2,151,149, dated March 2l, 1939.

It may be noted that, inthe circuit of' Fig. 1, it may be desirable inv som-e cases to replace the particular second separating circuit shown by one of the diode type. As to the :tirst separating circuit of the complete double separator combination, the particular type shown in Fig. l is generally preferred.

1 claim as my invention:

1. In a television receiver wherein synchronizing pulses and picture signals are received as a composite signal in which said pulses are of greater amplitude than the picture signals, electric discharge tube means for separating said synchronizing pulses from said picture signals,

l.ari amplifier and a second electric discharge tube means for separating'said pulses from said picf ture signals, said iirst means. said amplifier and -nizing impulses and picture signals are received as a compositesignal in which said pulses are of greater amplitude than said picture a iirst means including a vacuum tube having a grid for separating said pulses from said picture signals when said composite signal is impressed upon said grid with suiiicient amplitude and with the pulses of positive'polarity, an ampiiiier tube.

and a second means for separating said4 pulses from said positive signals. saidl rst means, said amplier, and said second means being connected in cascade in the order named.

3. The invention according to claim 2 characterized in that said ampliiler tube is ydesigned for working with approximately the same grid swing as is the vacuum tube of said iirst separating means.

4. The invention according to claim 2 wherein the vacuum'tube of said iirst separating means has a grid condenser and a Grid leak'resistor with such relative values that said vacuum tube is grid-leak biased due to said pulses driving its grid positive periodically.

. RANDALL C. BAILARD. 

